The winter storm can be traced back to two short wave troughs
that were moving across the western
U.S. on 12/23. One vortex was embedded in the southern branch of the jet
stream and located over the Desert Southwest region at 12 UTC on
12/23. The other shortwave trough was located in the northern
branch of the jet stream and located across the northern U.S. Rockies. By 12 UTC on 12/24,
the two systems were easy to identify in the
upper air analysis and the
water vapor imagery. The southern stream vortex was about to move into Texas and the northern
stream trough was moving east across the Dakotas.

Even as the coastal low pressure area moved away from central North Carolina,
the mid-upper low and associated dynamic forcing moved across
the southern Appalachians into the Mid-Atlantic coast on Sunday afternoon and
evening. The forcing for ascent with this feature reinvigorated the lingering
light snow and flurries that had persisted in the wake of the coastal storm.
Small snow grains grew to nickel to quarter-sized dendrites during the late
afternoon and evening hours, as the mid levels cooled and moisture grew deeper
in the minus 10 to minus 15 C layer (between 8-14 kft). This resulted in an
additional inch or so of accumulation across the North Carolina Coastal
Plain.

Snow Accumulation Map

Regional Snow Accumulation Estimate

MODIS Terra Satellite Image Showing Snow Cover from 2010/12/27

Forecast Challenges

This winter storm provided a significant amount of difficulty for forecasters as the model guidance shifted
the storm eastward and weakened it during the 00 UTC 12/24 and 12 UTC 12/24 runs. The difficultly was centered
around whether the northern and southern jet streams would phase which would dictate whether
the storm was weak and moved out to sea or whether the storm significantly intensified and
moved up the East Coast. The change in model guidance was frustrating for forecasters and
made communicating and explaining the potential threat to our users more difficult.

The erratic nature of the NWP guidance with this event will like be a topic of research for several
years to come. Researchers are suggesting that the large shift in the forecast track with the 00 UTC 12/24
and 12 UTC 12/24 runs may have originated from small initial condition uncertainties/errors in the
short wave trough in the central and southern Plains. Small initial condition differences with the
GFS and the ECMWF at 1200 UTC 12/24 appear to have amplified to large differences in the forecast
36-48 hours later.

The surface pattern during the event was consistent with many past significant winter
storms in central North Carolina with a storm system tracking northeast
across the northern Gulf Coast and the Southeast U.S. coastalong with an arctic air mass centered across the eastern
Great Lakes. The event followed a "Miller A" surface pattern in which a single well developed
low pressure system develops in the Gulf of Mexico and tracks northeast across Florida before
moving up the U.S. East Coast while rapidly deepening. "Miller A" events typically result in
a simpler precipitation pattern that includes corridors of snow and rain with a well defined rain-snow
line with little or no icing in the narrow transition zone.

A Java Loop
of surface analysis imagery from 00 UTC on December 24 through 12 UTC on December 27, 2010
shows the evolution of the event. Note that the surface low track is in a favored location
for significant winter storms in central North Carolina. In addition, the surface high pressure system
was of sufficient strength (around 1035 MB) but centered west of the preferred location in
the eastern Great Lakes region.

Satellite Imagery

Water vapor imagery was used to monitor the short wave troughs in both the northern and southern jet streams and
to identify the potential phasing of the jet streams. The animation below contains hourly water vapor imagery
from 0015 UTC December 24 through 1215 UTC December 27, 2010, a loop that the user can stop/start/change speeds
is available able below.

In the loop below, a southern stream vortex can be seen moving east from the Desert Southwest region across Texas
and to near the northern Gulf of Mexico through the morning of 12/25. At the same time, two short wave troughs
can be seen moving southeast across the Missouri Valley. The second short wave trough reaches northern Arkansas
by the morning of 12/25 with both troughs merging later that evening and initiating rapid cyclogenesis off
the Southeast and Mid-Atlantic coast.

A Java Loop of
water vapor imagery from 0015 UTC December 24 through 1215 UTC December 27, 2010 is available.

Southeast Regional Radar Imagery

Regional radar imagery shows the two different areas of precipitation that impacted
central North Carolina during the event. These features can be seen in a Java Loop of
Southeast regional radar imagery from 1158 UTC on December 25 through 1958 UTC on December 26, 2010.

The first area of precipitation which stretched southwest to northeast into western and northern North Carolina
during the morning and afternoon hors on 12/25. This area of precipitation was largely driven by mid and upper level features and produced several
inches of snow across western and northern North Carolina. The second area of precipitation which developed during the evening
on 12/25 and continued overnight into the morning on 12/26 was associated with the merging upper level troughs and the deepening
surface low off the coast. This precipitation was banded in nature and heavy at times and resulted in the majority of the
accumulation across central and eastern North Carolina.

TREND’s Predominant P-type Nomogram

The nomogram to the right shows the distribution of precipitation (p-type) TREND's as a function of partial
thickness values. Close examination of precipitation events over the past 30 years
accounts for the boundaries on the nomogram separating the various p-type areas.
Mid level thickness values increase from left to right along the x axis. Low level thickness
values increase from bottom to top along the y-axis.

This nomogram displays the observed thickness values from the 6 or 12 hourly RAOB's
at KGSO from 12 UTC on 12/23 through 12 UTC on 12/26. Thickness values preceding the event gradually warmed
through 12 UTC on 12/24 with a more significant low level warming during the afternoon of 12/24.
Thickness values began to cool after 00 UTC on 12/25 with the greatest cooling occurring in the
mid levels in the 850-700 mb layer which dropped 34 meters between 00 UTC on 12/25 through 00 UTC on 12/26;
while the low level thickness values in the 1000-850 mb layer dropped only 14 meters. This is indicative of falling
heights aloft while the low levels remained somewhat mild with little cool advection.

Light snow arrived at KGSO just after 16 UTC on 12/25 with surface temperatures in the upper 30s.
Diabatic cooling from evaporation allowed surface temperatures to quickly drop to 33 degrees after the
onset of precipitation. Subsequent diabatic cooling, likely from melting snow, allowed temperatures to fall to 32 degrees
after 3 or 4 hours of precipitation. During this time, the predominate precipitation type nomogram when using thickness values from
the 12 UTC 25 December KGSO RAOB and
the 00 UTC 26 December KGSO RAOB was trending from "measurable snow
with rain" toward "all snow." Greensboro officially received 5.8 inches of snow with the heaviest snow falling
during the late afternoon on 12/25 with a steady but mainly light snow falling overnight through the
late morning hours on 12/26.

High Resolution Model Reflectivity Data

Comparison of NCEP High Res WRF-NMM and WRF-ARW with Observed Radar Reflectivity

During this event, forecasters used hourly and 3 hourly output from the
NCEP High Resolution WRF NMM and ARW models along with the NCEP WRF-NMM run for the SPC,
the NSSL WRF-ARW run, and runs from the WFO RAH WRF-NMM simulations successfully. They found
the hourly and 3 hourly data to be very helpful in examining precipitation and QPF trends and in
identifying mesoscale features. For example, narrow progressive banded structures are easier to
identify and examine in hourly or 3 hourly output. These features get smeared out and
are difficult to resolve in 6 hourly output.

Recent research has shown that convection allowing high resolution NWP reflectivity
forecasts can be useful in the forecast process when used appropriately. In order to facilitate
the use of high resolution NWP reflectivity forecasts, the NWS Raleigh created the
High Resolution Model Reflectivity
Forecast Comparison Web Page. This pages allows users to easily compare
NCEP High Resolution WRF NMM and ARW models along with the NCEP WRF-NMM run for the SPC,
the NSSL WRF-ARW run, and runs from the WFO RAH WRF-NMM simulations. A snap shot of the web
page is shown below.

Hourly 4 inch (0.1m) Soil Temperatures

The image below (click on it to enlarge) shows the hourly 4 inch soil
temperatures at 8 locations across central North Carolina from midnight on
12/24 through midnight on 12/30, 2010.

First, note that the 4 inch soil temperatures were rather cold on 12/24,
with soil temperatures at most locations dropping into the mid 30s or colder on the morning of
12/24. Despite mostly sunny skies during the day and
high temperatures mainly in the lower to mid 40s,
soil temperatures only warmed slightly from the morning minimums with a diurnal
spread of between 4 and 8 degrees on 12/24. The nocturnal cooling of the soil was likely reduced by the
warmer air temperatures compared to previous nights. Air temperatures on the morning of 12/25
ranged in the lower to mid 30s with mostly cloudy skies.

Precipitation spread into the western Piedmont during the late morning hours on 12/25.
The precipitation was responsible for the early peak in the hourly soil temperature plot for stations
NCAT (NC A&T Research Farm) and HIGH (UNCG Lindale Farm Station) which are both located in Greensboro. Note that the max soil temperature for these two locations occurred around
noon local time. The other stations reached their maximum soil temperature around
the more typical time of 400 PM local time.

In general, most locations had a rather steady decline in soil temperatures as
precipitation fell across central North Carolina. Note that both LILE (Lilesville) and
HAML (Hamlet) had a more aggressive decline in soil temperatures between 200 AM and
600 AM. This decline appears coincident with a period of heavier precipitation which
cooled the boundary layer and allowed the rain to quickly change to snow and accumulate
during the pre dawn hours. It is also worth noting that there was very little diurnal
change in soil temperatures on 12/26 as snow accumulated across most of central
North Carolina through at least midday

On 12/27, the soil temperatures at most observing locations in central North Carolina
showed very little diurnal change. This was not too surprising given
the fresh snow cover across the area. Note that there was one exception, LILE
which had around a 3 degree diurnal change in soil temperatures during the day. South-central
NC where Lilesville is
located, received around 4 inches of snow during the storm which was
on the low end of the general 4 to 12 inches of accumulation that fell across central NC.
In addition, high temperatures on
12/27th approached 40 degrees in Lilesville which resulted in some additional melting.

By 12/28, sunshine along with daytime temperatures well into the 40s
resulted in considerable melting and allowed the diurnal spread in soil temperatures to increase. The diurnal spread was greatest
at LILE, HAML, and in Greensboro at the NCAT and HIGH where the snow melted more significantly compared to
CLIN (Clinton), GOLD (Goldsboro), CLA2 (Clayton) and SILR (Siler City) where the more significant snows were slower to melt.

Past experience has shown that when the max soil temperature during the day preceding a snow fall are
in the lower 40s or colder and given modest snow rates with surface temperatures at or near freezing,
the snow can be expected to accumulate. Max soil temperatures in most of central North Carolina were
in the lower to mid 40s on 12/24. During this event, the snow was slower to accumulate than then
12/16, 2010 event which had very cold temperatures preceding and during that event which allowed
the snow to readily accumulate, even on roadways. During this event, boundary layer temperatures in the eastern
Piedmont and Coastal Plain were near or just above freezing which initially limited accumulations.
Eventually, heavier precipitation and cooling from melting allowed air temperatures to fall to around or below freezing
and allow the snow to accumulate more efficiently.

CoCoRaHS is a grassroots volunteer network of weather observers working together to measure and map precipitation
(rain, hail and snow) in their local communities. By using low-cost measurement
tools, stressing training and education, and utilizing an interactive web-site,
CoCoRaHS aims to provide the highest quality data for natural resource, education
and research applications. The only requirements to join are an enthusiasm for
watching and reporting weather conditions and a desire to learn more about how
weather can effect and impact our lives. North Carolina joined the CoCoRaHS network in 2007.
For more information, visit the CoCoRaHS web site at
www.cocorahs.org.

The CoCoRaHS Web page provides the ability for CoCoRaHS observers to see their observations
mapped out in "real time", as well as providing a wealth of information for
our data users. The snow accumulation maps
from the CoCoRaHS web site (shown below) were a great resource for WFO RAH.
The CoCoRaHS observers in central North Carolina were notified of the potential storm in advance
and encouraged to report during the event. Observations from the CoCoRaHS observers were excellent and
very timely.

CoCoRaHS Intense Snow Report

NWS Raleigh received 45 CoCoRaHS Intense Snow Reports during this winter storm. An
example is shown below with all of the reports available here.
The intense snow reports are extremely helpful since it can be difficult to get accurate snow
accumulation reports during the event and especially at night. Several of the Intense Snow
Reports were received late in the evening or overnight
when it is very difficult to get reliable information. The value of these reports cannot be overstated.

Select Photos of the Winter StormPhotos courtesy of William Wilson and Jonathan Blaes

Final Thoughts and Lessons Learned

Forecasters were challenged with evolving NWP guidance which had great difficulty in predicting the storm
intensity and location, especially during the 00 UTC and 12 UTC 12/24 runs. The difficultly was centered around whether
the northern and southern jet streams would phase which would dictate whether the storm was weak
and moved out to sea or whether the storm significantly intensified and moved up the East Coast.

Predicting the phasing of the northern and southern jet streams can be problematic. For this event, the
NWP guidance struggled with an event that was 36-60 hours out, while the short wave troughs were
entrenched in the land based RAOB network. The careful approach used by forecasters at WFO RAH was to be
cautious about jumping on the event in the long term but rather gradually step into or trend the forecast toward
a stormier solution. When the guidance backed away from as significant storm, forecasters did not make wholesale changes
but rather trend the forecast. As the event drew closer, forecasters used observational data and pattern recognition to jump on
the storm and issue timely winter storm watches. Despite the changes in model and HPC guidance, forecasters did not sway making large day to day
changes with the forecast. We began conservatively 6-7 days out, introduced the threat of a winter storm,
trended toward a drier solution 2-3 days out, and then trended back toward a winter storm on Christmas Eve.

Two web based tools were helpful for forecasters while evaluating model trends:

The limitations with deterministic forecasts and guidance along with the utility of probabilistic
forecasts were very obvious with this event. A few days ahead of the storm
we had a high confidence of some light snow, moderate confidence of some decent snow accumulations although the
locations were unclear, and small but non zero chances of a very big storm. A single deterministic forecast
of 2-4 inches doesn't really help in these situations.

Numerous briefings (example from 600 AM 12/25) were
provided to local emergency managers and decision makers during the event
via the NWS
Raleigh Briefing Web Page and via other online conferencing software.
The ability to share information with users who may be at home or away from the office was invaluable.

During this event, forecasters used hourly and 3 hourly output from the
NCEP High Resolution Window WRF NMM and ARW models along with the NCEP WRF-NMM run for the SPC,
the NSSL WRF-ARW run, and runs from the WFO RAH WRF-NMM simulations successfully. They found
the hourly and 3 hourly data to be very helpful in examining precipitation and QPF trends and in
identifying mesoscale features. For example, narrow progressive banded structures are easy to
identify and examine in hourly or 3 hourly output. These features get smeared out and
difficult to resolve in 6 hourly output.

WFO RAH issued 274 LSR’s from 300 PM on December 25th through 1159 PM on December 26th.
In addition, there were 18 PNS’ issued from 730 PM on December 25th through 1200 AM on December 27th
with PNS’ issued nearly every hour from midnight to noon on December 26th. Users were very happy to get snow
accumulation reports in real time via the LSR product which allowed them to plot reports directly and in
real time along with complete hourly summaries in the PNS.

NWS Raleigh received 45 CoCoRaHS Intense Snow Reports during this winter storm.
These reports are especially important since it is very difficult to get accurate snow accumulation
reports late at night. The value of these reports cannot be overstated.

Acknowledgements

Many of the images and graphics used in this review were provided by parties outside of WFO RAH.
CONUS snowfall estimates provided by the National Operational Hydrologic Remote Sensing Center.
MODIS imagery provided by the Space Science and Engineering Center - University of Wisconsin-Madison.
Model comparison charts, comparison of ensemble forecast points, and comparison of HPC vs. SREF snowfall probabilities provided by SUNY Stony Brook CSTAR project and Dan Petersen.
The surface analysis imagery was obtained from the Hydrometeorological Prediction Center.
Satellite data was obtained from the National Center for Atmospheric Research.
The upper air analysis images and Skew-T diagrams were obtained from the Storm Prediction Center.
The University of Wyoming, and the Earth System Research Laboratory - Global Systems Division.
AMDAR aircraft sounding data was obtained from the Earth System Research Laboratory - Global Systems Division.
Surface observations provided by the University of Wyoming.
Radar imagery was obtained from the National Weather Service web site.
Ground temperatures and adjacent air temperature data provided by CRONOS from the N.C. State Climate Office.
CoCoRaHS maps were provided by the CoCoRaHS organization.
Photographs courtesy of William Wilson and Jonathan Blaes.